Flexible pipes of the above type are e.g. used for the transport of oil and gas at large or varying sea depths, e.g. from under water wells to installations at the surface of the sea where the oil or gas is refined or sent on for processing.
The construction of pipes of this kind is e.g. known from JP-3265781, wherein a flexible armoured pipe is described. The armouring is surrounded by a tight sheath which prevents the penetration of fluids from the surroundings to the armouring of the pipe whereby the armouring has to absorb forces from internal as well as external pressures.
A problem with this type of pipe is that the armouring layer—because of the tight sheath around it—has to be able to withstand the chemical activity from fluids escaping from the interior of the pipe, e.g. by diffusion and thereby accumulates around the armouring layers.
On the other hand, U.S. Pat. No. 4,402,346 describes a pipe wherein the armouring is surrounded by a permeable outer sheath, which has the advantage that aggressive gases diffusing out through the liner will not be accumulated in the volume around the armouring profiles.
A problem with both types of pipes is that the outer sheath—permeable or not—may hinder a radial expansion of the wound armouring layers which may result in the creation of critically large compressive forces in the armouring layer when the pipe is subject to an outer hydrostatic pressure. These compressive forces may lead to bursting of the outer sheath or to instability of the armouring profiles resulting in damaging deformations.
One solution to this problem is to make the outer sheath in the form of a braided hose as it is known from ropes and some electro-hydraulic cables. Such a braided hose may e.g. be made from threads comprising polyester fibre, aramide fibre, polyethylene mono-filaments, or metallic filaments. The threads may be spun, twisted or braided into chords or assembled in tapes.
A problem with the braiding of an outer sheath for a flexible pipe is that the circumferences of typical flexible pipes are much larger than those of even large ropes (e.g. for anchors) and cables, and which therefore require very large machines compared to those of the rope and cable industry. Further, the velocity of the braiding process is considerably lower than what is typical for winding processes in connection with the manufacture of flexible pipes and therefore not easily integrated in a normal manufacturing flow.